1. Field of the Invention
The present invention relates to a device for purifying the exhaust gas of an internal combustion engine.
2. Description of the Related Art
The exhaust gas of an internal combustion engine and, particularly, of a diesel engine contains harmful particulates comprising carbon as a chief component, and it has been desired to lower the amount of particulates emitted into the atmosphere. It has therefore been proposed to dispose a particulate trap in the exhaust system of the diesel engine as a filter for trapping the particulates. Such a particulate trap could become a large resistance against the exhaust gas as the amount of trapped particulate increases. It therefore becomes necessary to regenerate the particulate trap itself by burning the trapped particulates.
When the exhaust gas temperature becomes high in high-engine-load and high-engine-speed operating conditions, and the like, the trapped particulates ignite spontaneously and the particulate trap can be regenerated. However, there is no guarantee that the high engine load and high engine speed operation conditions are frequently carried out. Generally, therefore, the regeneration treatment is executed by arranging a heater or oxidation catalyst in the particulate trap, and energizing the heater or supplying unburned fuel into the oxidation catalyst to cause the regeneration.
It therefore becomes necessary to determine the timing for regenerating the particulate trap. When the determined timing for regeneration is too early, the regeneration treatment is executed unnecessarily causing a problem that the battery must large or a large amount of fuel is consumed. When the determined timing for regeneration is too late, on the other hand, the resistance against the exhaust gas very greatly increases in the engine exhaust system causing a large drop in the engine output.
It is therefore desired to correctly determine the timing for regenerating the particulate trap. It has, for example, been proposed to determine the timing for regeneration relying upon the fact that an amount of trapped particulate increases with an increase in the distance which the vehicle has traveled. However, a distinct difference occurs in the amount of trapped particulate depending upon the operation conditions over a predetermined distance which the vehicle has traveled with this method, therefore, it is not possible to correctly determine the timing for regeneration.
Japanese Unexamined Patent Publication (Kokai) No. 3-41112 proposes the determination of a timing for regenerating the particulate trap by comparing the amount of measured fresh intake air with a reference value for every engine operation condition relying upon the fact that the amount of the fresh intake air decreases with an increase in the amount of trapped particulate.
By the way, in order to decrease an amount of NOx which is a harmful component produced by combustion, there has been widely known an exhaust gas recirculation (EGR) device that lowers the temperature of combustion by recirculating a part of the exhaust gas into the cylinders, and many internal combustion engines have been equipped with such an EGR device. The above-mentioned determination of timing for regeneration is rendered relatively correctly. To apply this to an internal combustion engine equipped with the EGR device, however, it becomes necessary to interrupt the recirculation of exhaust gas at the determination. In the normal engine operations, therefore, a large amount of NOx is produced during this period.
It is therefore an object of the present invention to correctly determine the timing for regenerating the particulate trap arranged in the exhaust system of an internal combustion engine equipped with an EGR device by correctly estimating an amount of particulate trapped by the particulate trap without increasing an amount of produced NOx.
According to the present invention, there is provided a device for purifying exhaust gas of an internal combustion engine comprising:
a particulate trap disposed in the engine exhaust system;
an exhaust gas recirculation passage communicating the upstream of the particulate trap in the engine exhaust system with the engine intake system;
a control valve for controlling the amount of exhaust gas recirculated through the exhaust gas recirculation passage to be an optimum amount in accordance with an engine operation condition;
fresh air amount detection means for detecting an amount of fresh air introduced into the engine intake system during a fuel-cut; and
an estimation means for estimating an amount of particulates trapped by the particulate trap on the basis of the amount of fresh air detected by the fresh air detection means after the control valve is opened to a predetermined opening degree.
According to the present invention, there is provided another device for purifying exhaust gas of an internal combustion engine invention comprising:
a particulate trap disposed in the engine exhaust system;
an exhaust gas recirculation passage communicating the upstream side of the particulate trap in the engine exhaust system with the engine intake system;
a control valve for controlling the amount of exhaust gas recirculated through the exhaust gas recirculation passage to be an optimum amount in accordance with an engine operation condition;
a pressure differential detection means for detecting directly or indirectly a pressure differential between the upstream side and the downstream side of the particulate trap during a fuel-cut after the control valve is fully closed and a throttle valve disposed on the engine intake system is fully opened;
an estimation means for estimating an amount of particulate trapped by the particulate trap on the basis of the pressure differential detected by the pressure differential detection means.
The present invention will be more fully understood from the description of preferred embodiments of the invention set forth below, together with the accompanying drawings.